Engine 91 LX Rhythmic Thumping sound and high idle

VSK2033 · Jul 23, 2019

Hi, so the past weekend I noticed cylinder 8 failed the balance test so I did a compression test on all cylinders, checked spark plugs plus wires and took intake off to check injectors and everything came out good. Once I put everything back together and started it up without the IAC plugged in, the idle hung for a second at about 1k rpms and then shot up to 3k rpms and stayed there and at the same time a noticable thumping sound started coming from somewhere inside the engine and was pretty rhythmic where it thumped every few seconds. I am a little lost as to what this could be and was wondering if anyone else experienced anything like this.

General karthief · Jul 23, 2019

What happens when you start it with the IAC plugged in?

VSK2033 · Jul 23, 2019

General karthief said:
What happens when you start it with the IAC plugged in?

I tried that last night and it immediately idles at 3k rpms and will hold it there until the car is turned off, without it plugged in, it would idle at about 1k rpms and after like 5 seconds it felt like someone pushed down the pedal cause it would jump up to about 2300-3000 rpms

General karthief · Jul 23, 2019

Some things to check. There is a checklist for this but I feel it is a reassembly issue.
throttle cable out of whack, hung up or base idle out of adjustment.
vacuum leak?
have you reset the computer?
if you didn't have the idle problem before it likely something out of whack when you installed the upper intake.
here is the checklist that will help you also.

Help me create the "Surging Idle Checklist"

Updated 26-Oct-2019 to clarify creeping idle RPM increase as the car is being driven. Many of you are familiar with the "Cranks OK, but No Start Checklist for Fuel Injected Mustangs" checklist and the No Crank Checklist. They are very effective in finding and eliminating the problems by use of...

forums.stangnet.com

VSK2033 · Jul 23, 2019

General karthief said:
Some things to check. There is a checklist for this but I feel it is a reassembly issue.
throttle cable out of whack, hung up or base idle out of adjustment.
vacuum leak?
have you reset the computer?
if you didn't have the idle problem before it likely something out of whack when you installed the upper intake.
here is the checklist that will help you also.

Help me create the "Surging Idle Checklist"

Updated 26-Oct-2019 to clarify creeping idle RPM increase as the car is being driven. Many of you are familiar with the "Cranks OK, but No Start Checklist for Fuel Injected Mustangs" checklist and the No Crank Checklist. They are very effective in finding and eliminating the problems by use of...

forums.stangnet.com

Yeah the idle was fine before, we had surging issues in the past and that checklist is what I used to help smooth things out, my biggest concern is the loud thump sound that I have never heard before because I haven't really had a chance to get idle sorted out because I get nervous when it starts to thump so I turn it off. Its a very deep sounding thump coming from somewhere inside the engine, At I first thought it was spark knock or something of the sort but its not a pinging sound or rattling metal. Is it possible that a major vacuum leak or something could cause this?

General karthief · Jul 23, 2019

Can't answer that without standing next to it in person. It may be possible that your air pump or the associated plumbing has developed a leak,

Mustang5L5 · Jul 23, 2019

You took the intake off...now the car idles high.

You have a vacuum leak somewhere

Engine 91 LX Rhythmic Thumping sound and high idle

VSK2033

Member

General karthief

wonder how much it would cost to ship you a pair

VSK2033

Member

General karthief

wonder how much it would cost to ship you a pair

Help me create the "Surging Idle Checklist"

VSK2033

Member

Help me create the "Surging Idle Checklist"

General karthief

wonder how much it would cost to ship you a pair

Mustang5L5

That is…until I whipped out my Bissell

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Engine 91 LX Rhythmic Thumping sound and high idle

Member

wonder how much it would cost to ship you a pair

Member

wonder how much it would cost to ship you a pair

Member

wonder how much it would cost to ship you a pair

That is…until I whipped out my Bissell

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